Heat exchanger



FIG. 1 is a front elevational view of a heat exchanger showing my new design;

FIG. 2 is a rear elevational view thereof;

FIG. 3 is a top plan view thereof;

FIG. 4 is a bottom plan view thereof;

FIG. 5 is a left side elevational thereof, the right side being a mirror image;

FIG. 6 is a front perspective view thereof, the rear being a mirror image.

A rear perspective view thereof is the same as a front perspective view thereof. 

The ornamental design for a heat exchanger, as shown and described. 